1. Field of the Invention
The present invention relates to an optical connector for coupling the conductor of an optical fiber cable to that of another optical fiber cable or to an optical waveguide or others of an optical device, more particularly, is concerned with means for suppressing increases of a transmission loss, which result from bends of the connection between the optical fiber cable and the optical connector plug.
2. Related Background Art
To couple the conductors of single mode type optical fiber cables, recently removable optical connectors are dominantly used. This trend is more conspicuous in internal optical wirings, and optical wiring to machines and instruments.
Such optical fibers generally require to achieve small coupling losses, easy removal, small sizes and lightness, mass-production at low costs, etc. The inventors of the present invention have made various studies on the improvement of the transmission characteristic of the optical fiber cables and have been confronted with the problem of transmission losses due to bends of the connections between optical connector plugs and the optical fiber cables.
That is, in the case that an optical connector plug is used in a horizontal arrangement, the optical fiber cable droops due to its own weight. And, when the optical fiber cable is downwardly pulled for maintenance and inspection, the optical fiber cable is caused to bend at an acute angle at the connection with the optical connector plug. This bend increases the transmission loss of the optical fiber. The inventors of the present invention noted this problem. They have made various experiments and analyzed points of the problem. The points of the problem will be explained below with reference to a drawing.
FIG. 1 is a sectional view of the conventionally used optical connector plug. This optical connector plug 100 includes a ferrule 102 to be connected to one end of an optical fiber conductor 104 of an optical fiber cable 106. This ferrule 102 is housed axially slidably in a plug body 108. When the plug body 108 is engaged in an alignment member 110, the ferrule 102 of the optical connector plug 100 is brought into alignment with a ferrule 112 of another optical connector plug 114, and the conductors of the optical fiber cables are coupled with each other. A stop ring 116 for prohibiting the ferrule 102 from coming off the plug body 108 is inserted and secured in the rear end portion of the plug body 108 (the right end in FIG. 1). A cable retention ring 118 is connected with the end portion of the stop ring 116.
The optical fiber cable 106 is extended outside through the central hollows 120 of the stop ring 116 and the cable retention ring 118. The rear end portion of the cable retention ring 118 has a reduced diameter and is covered with the jacket 122 of the optical fiber cable 106. The jacket 122 is fastened to the end portion of the cable retention ring 118 by a fastening ring 124 surrounding the jacket 122.
The cable retention ring 118, and a portion of the optical fiber extended out of the cable retention ring 118 are covered with a strain relief boot 126. The boot 126 is for the protection of the boundary 128 between the retention ring 118 and the optical fiber cable 106.
The strain relief boot 126 is conventionally made of silicone rubber or elastic plastics, and is formed in one piece. The section of the boot 126 is as shown in FIG. 1. Its forward portion 126a is as thin as about 0.55 mm, and the rearward portion 126b is formed thicker and is tapered to decrease its thickness toward the rear end portion. A difference in thickness between the forward portion 126a of the boot 126 and the rearward portion 126b thereof is because there is a difference between a maximum outer diameter of the fastening ring 124 and an outer diameter of the optical fiber cable 106, and the fastening ring 124 is formed of a harder metal which allows the forward portion 126a to be relatively thin.
However, the above-described conventional boot 126, which is soft, adversely allows the optical fiber cable 106 to the rearward portion 126b, which should function to rearwardly distribute a curvature of the optical fiber cable, to form a downward acute angle at the proximal end portion 126c of the rearward portion 126b of the boot 126 as shown in FIG. 2 when the optical fiber cable 106 is subjected to a downward load, such as a pull by a hand, in a maintenance and inspection operation. The bend is concentrated on the proximal portion 126c on the cantilever principle, because the rearward portion 126b itself is too flexible, has a small diameter, is long, and has a relatively small variation of thickness. Under a downward load of the optical fiber cable 106 of above about 0.3 kgf, the rearward portion 126c cannot prevent the acute bend of the optical fiber cable 106. Therefore, the transmission loss of the optical fiber cable 106 increases. As described above, a problem of the conventional optical connector plug is that its structure is not optimum to allow the optical fiber to bend at blunt angles which suppresses increases of its transmission loss.